Use asynchronous prefetching of multi-page ResultSets in CassandraRDD to reduce waiting for Cassandra query results.
Merge pull request #331 from datastax/wip-prefetching

Performance: Use asynchronous prefetching of multi-page ResultSets in CassandraRDD to...
Make token range start and end be parameters of the query, not part of the query template.
Therefore a query template does not depend on the actual token range, and need not be prepared separately for every task.
Perform test preparation for CassandraJavaUtilSpec only once. Write to different tables to avoid data races.